Owing to their exceptional properties and diverse capabilities for chemical functionalization and hierarchical materials integration, nanostructures are sought for a myriad of applications ranging from electronic devices to structural composites. These nanostructures include single- and multi-wall carbon nanotubes, semiconducting, metal, and oxide nanowires, and various superlattice and branched heterostructures.
For high-performance energy conversion and storage (e.g., photoelectric, electromechanical, electrochemical), energy carriers must be generated and transported at high efficiency and high density. Engineering of nanostructures to efficiently generate carriers upon a desired excitation (e.g., mechanical, optical, thermal), along with organization of nanostructures to enable efficient transport is an extremely promising approach in this regard. For example, an individual silicon nanowire consisting of a p-type core and a n-type shell generates a photoelectric current, and nanowire arrays can be configured as piezoelectric energy harvesters or dye-sensitized solar cells (Tian et al., Nature 449:885, 2007; Wang and Song, Science 312:242, 2006; Law et al., Nature Mater. 4:455, 2005). The high surface-to-volume ratios of nanostructures impart high sensitivity to adsorbed charges and molecular species, as exploited by resistive and capacitive chemical sensors using tangled CNT networks, batteries using oxide nanotubes, and mass sensors using functionalized thin-film cantilevers. Overall, large numbers of nanoscale junctions and interfaces enable integrated and efficient energy conversion storage in small-scale devices.
The piezoelectric properties of zinc oxide nanowires (ZNWs) have recently been demonstrated, along with extraction of electrical power from mechanical deformation induced by deflecting individual ZNWs or by coupling a ZNW array to ultrasonic excitation (Wang and Song, supra; Wang et al., Science 316:102-5, 2007; and related publications). In these previous devices, the ZNWs are actuated by external contact using a metal-coated AFM tip or a zigzag-etched top electrode. As the actuator strains a nanowire by deflecting the tip, a piezoelectric voltage is generated in the nanowire. Discharge occurs when the actuator contacts the negative-potential side of the nanowire. The requirement for external actuation can be a significant drawback, because it can limit the density of nanowires (spaced according to the electrode topography) and the number of contacts per nanowire (only one at the tip). Additionally, assembly of the top electrode can be difficult, as it must move in a cyclic fashion against the tips of the nanowires.
Scalable manufacturing of energy conversion devices demands both engineering of individual nanostructures (e.g., size, composition, crystal structure), and organized assembly (e.g., alignment, spacing) and addressing of large numbers (>109/cm2) of nanostructures. While progress is being made on both fronts, the ability to engineer individual structures far outpaces the ability to assemble structures in a hierarchical or directed fashion.
Microsystems are vital technologies for military reconnaissance, security, and warfighter performance. These include chemical and inertial sensors, high-performance electronics and RF devices (e.g., tracking/locating tags, resonators, antennas), and implantable/wearable biosensors. Integrated energy harvesting and storage capabilities may be useful for long-term, remote, and self-powered operation of these devices. Considering limitations of state-of-the art nanomanufacturing techniques, a need exists to advance capability for new nanomaterial architectures for energy harvesting and storage in MEMS/NEMS devices.